1. Field of the Invention
This invention relates to radio frequency ranging apparatus. More particularly, the apparatus relates to an improved two-way ranging system for determining the range between a ground station and a remote station without the requirement for turnaround coherency.
2. Description of the Prior Art
This invention is an improvement in the apparatus shown and described in application Ser. No. 180,737 filed Aug. 25, 1980, now U.S. Pat. No. 4,347,009 for a Noncoherent Two-Way Ranging Apparatus. This previous application describes apparatus and a method of performing two-way range measurements in approximately half the acquisition time of prior art coherent systems. This previous application explains that ranging systems have been conveniently divided into classifications depending on the coding employed. One of the most desirable and secure types of codes involves the utilization of a pseudonoise generated spread spectrum code. When such codes are made non-linear they can be very secure means of transmitting codes and data. Such codes are desirable because they are capable of transmitting information in a sequence of patterns that resemble a random noise pattern. Such random noise patterns are difficult to distinguish from random occurring signals. Pseudonoise generated spread spectrum codes have been employed to transmit codes and data for a long enough time that sophisticated computer analysis techniques have been developed for deciphering the unique pattern sequences. When attempts are made to intercept and analyze spread spectrum generated codes, the period of intercept is preferably made long enough to build up a reliable analysis of the intercepted information. The number of unique patterns in a pseudonoise generated spread spectrum code are 2.sup.n-1 in length and "n" is an integer. This code sequence is normally made long enough to cause difficulties in intercepting and deciphering the sequence and as a result increases the acquisition time for receiving the information.
In a coherent ranging system, a pseudonoise generated signal is transmitted from a first station to a second station and the second station employs means for tracking and locking on to the signal. The time initially required for the pseudonoise generator in the receiving station to track and lockon to the incoming pseudonoise generated signal is referred to as acquisition time. It is only when the code is acquired and locked on that the pseudonoise generated information at the receiving station is synchronized with the coded signal being received so that transmission of data is possible.
One of the problems with spread spectrum codes is that the acquiring station must examine each unique signal for a period of time and build up reliability of the signal being received. When the pseudonoise generated signal being received is out of synchronization with the pseudonoise generator in the receiving station, the control logic at the receiving station must step the tracking system generator to another unique signal in the spread spectrum code. The receiving station must continue to step the tracking system generator until the received pseudonoise generated signal matches the pseudonoise generated signal in the receiving station. Once the receiving station system is locked on to the transmitted signal it is said to be coherent. In some of the prior art ranging systems the received signal is processed in the remote station and retransmitted back to the ground station where the retransmitted signal must be reacquired to provide coherent lock-on of the retransmitted signal. Thus, it will be understood that in two-way ranging systems when the remote station locks on to the receive signal and retransmits the receive signal to the ground station so that the ground station can reacquire and lock-on to the transmitted signal that the two-way ranging system is said to be completely coherent. The aforementioned application's Ser. No. 180,737 describes a ranging system in which the spread spectrum transmitted coded signals are tracked by a tracking generator in the remote station. The remote station pseudonoise generator produces a replica of the original pseudonoise generated code. This tracking generator locks on to the transmitted signal and thus requires acquisition time in one direction. Since this previous application ranging system did not retransmit the acquired signal there was no requirement for two-way or turn-around coherency. This previous application achieved noncoherency by employing a second master clock and a second ranging system in the remote station. The range determined at the remote station was transmitted to the ground station in order to make a determination of the two-way range. This transmission of range data was made over the communications link.
The present invention improvement completely eliminates the need for a second master clock and a second ranging system in the remote station yet provides an accurate two-way ranging system which operates in the same or less acquisition time than the aforementioned application Ser. No. 180,737 apparatus. Before this previous application apparatus was provided, two-way ranging had been performed through coherent turn-around of a marker signal. In coherent turn-around systems, the receive spread spectrum signal is demodulated employing a pseudonoise generated replica signal at the remote station which must be synchronized and in phase with the receive pseudonoise generated waveform signal. The received marker signal is processed (i.e., turn-around) and retransmitted back to the ground station where it is demodulated employing a pseudonoise generated replica signal of the code being retransmitted. This pseudonoise generated replica sequence signal must also be demodulated by employing a pseudonoise generated replica which is locked on to the retransmitted waveform from the remote station. When coherent turn-around systems are employed for making ranging measurements, the remote station must first acquire and lock on to the original signal and after the remote station has acquired the ground station signal, it may then retransmit the processed signal back to the ground station which again requires reacquisition of the retransmitted signal. Coherent turn-around systems require that both stations be locked on to the waveform being transmitted from the other station, thus, the acquisition time for coherent turn-around systems require substantial acquisition time.
It will be understood that range measurements between stations can be calculated by measuring the time an electromagnetic signal takes to travel from a ground station to a remote station and make the return trip to the ground station. With complete knowledge of the delays encountered by the transmitting equipment and the receiving equipment in the two stations an accurate range determination can be made. It is well known that the delays in the equipment can be accurately measured before the remote station is placed in operation.
It would be extremely desirable to provide a ranging system which is secure and is not easily jammed. Further, it would be desirable to provide a ranging system which employs simple and reliable components of the type already employed in ranging systems and which would be arranged to operate in a manner which reduces the acquisition time.